1. Field of the Invention
In at least one aspect, the present invention is related to erasable ink compositions that substantially simulate the appearance of a graphite pencil, and more particularly to erasable ink compositions that substantially simulate the appearance of a graphite pencil and are suitable for use in replaceable cartridges.
2. Background Art
For a number of years, there has been a desire to produce a pen that produces an erasable, pencil-like trace. Although a number of erasable ink compositions are known, attempts to make erasable ink compositions that substantially simulate the trace of a pencil have been plagued by difficulties related to the poor quality of the trace produced and to poor erasability. Notwithstanding the problems associated with the pencil-simulating compositions, ink compositions that are erasable with a typical pencil eraser are known. Generally, such compositions maintain erasablity for at least several hours.
The erasable ink prior art includes erasable ink compositions that are both water-based and non-water based. The non-water based erasable ink formulations minimally contain a rubber, a pigment, and at least one solvent. For example, U.S. Pat. Nos. 4,390,646 and 4,389,499, each disclose an erasable ink composition which includes a thermoplastic block copolymer, pigments, and a solvent. Preferably the copolymers are a mixture of radial and linear block copolymers. Similarly, U.S. Pat. No. 4,721,739 (the '739 patent) discloses an erasable ink composition that contains a block polymer, a plasticizer, a lubricant, a colorant (i.e., a pigment), and a resin. Suitable plasiticizers as set forth in the '739 patent are aliphatic esters such as butyl stearate and dioctyl adipate; or aromatic esters such as dioctyl phthalate. These esters have a moderate molecular weight that is greater than 250.
The particular choice of pigments is important in maintaining the erasability of traces formed by the erasable ink composition. Moreover, the selection of an appropriate rubber (block polymer) dissolved in a solvent is such that when the solvent evaporates the ink viscosity increases to the point where further penetration of the ink to the paper is minimized. Unfortunately, these compositions are often plagued by clogging of the writing implement (i.e., the pen) over extended periods of time of non-use. This phenomenon tends to be somewhat worse when the ink is exposed to a humid environment. Accordingly, U.S. Pat. No. 4,629,748 (the '748 patent) provides at least a partial solution to this problem by disclosing the utility of various dispersants in erasable ink compositions which contain rubber and non-aqueous solvents. Specifically, the '748 demonstrates the use of alkyl phosphate ester, amines, and oxyethylene-oxypropylene block copolymers.
Similarly, numerous water-based erasable ink compositions exist in the prior art. Typically, such aqueous compositions include a block copolymer such as styrene-butadiene and a pigment. Generally such compositions also include one or more additives such as releasing and spreading agents, antioxidants, surfactants, gelatinizers, lubricants, and various waxes.
Although many of the various prior art erasable ink compositions generally work to some degree, there is still a need for an improved erasable ink composition that produces tracings that substantially mimic the tracings of a pencil without substantially clogging the writing implement and without substantial breaks in the formed tracings. Moreover, there is also a need for a formulation that is capable of producing tracings that are for erasable for one or more days instead of the few hours which is typical of the prior art formulations.